Know the book well. You should know the characters, the plot and various themes of the book. If you wrote the book, you will likely know it like the back of your hand. If you read it, it helps to take notes while reading so you can remember key points in the story.
Map out a basic outline of the story. You can create this as a chart or just random notes. This will help to organize your thoughts before you begin writing the synopsis. If you seem to be missing any key points in the plot, refer back to your original notes and confirm you have the details correct.
Start your synopsis with an interesting sentence that keeps the reader wanting more. You want the synopsis to be engaging from the first sentence. If you attempting to sell your book, you want agents, editors and publishers to keep reading. Keep in mind that they read hundreds of these weekly so yours should be catchy to stand out from the pack.
Detail the main plot in your synopsis. You should describe all the main characters and major points in the story. It is not important to outline every single detail so skip minor subplots. If there are one or two major subplots, you can include a brief summary of those.
Write your synopsis in the present tense. The synopsis should be active, like the story. Avoid writing in a passive voice.
Include specifics in your story. Avoid generalities that distance the reader. Specifics help to get the reader involved in the synopsis. For example, rather than describing your main character as a doctor, describe the specific type of doctor he is and the types of patients he treats.
Keep it brief. Remember a synopsis is a summary so don't get bogged down by including every detail of the book. Brevity is often best.
Read and edit your synopsis. It will often take several drafts before you get the synopsis in its best form. As you read it, ask yourself if there are any unanswered questions. If there are, edit it and answer those questions in your rewrite.